Reverse osmosis systems are manufactured using a variety of techniques and it can sometimes be confusing to understand the differences between the different types of reverse osmosis systems.

To begin, it is important to understand reverse osmosis and its components in order to fully understand the information below.

So, without going into detail, reverse osmosis systems are membrane systems that work like other types of filtration. Simply put, a pressure greater than the osmotic pressure is applied to move water through the system and across the osmotic membranes. To better understand how reverse osmosis works, we suggest you read this article:

For practical purposes and in order to fully understand the topics discussed below, it is important to know and understand certain terms:

  1. Permeate: it qualifies as a fluid that has passed through a semipermeable membrane. In other words, permeate refers to the purified water stream.
  2. Concentrate: refers to the fluid containing the contaminants/substances that have been retained by the membrane(s).
  3. Osmotic pressure: refers to the pressure that must be applied on the solution side to stop the movement of the fluid when a semipermeable membrane separates the two mixtures.


What is a reverse osmosis array?

Commonly referred to as an array, the network of a reverse osmosis system refers to the physical arrangement of the membrane housings and membranes. The array of a reverse osmosis system is usually expressed in these types of expressions:

  • For a two-stage system, 4 : 2
  • For a three-stage system, 8 : 4 : 2
    • Note that the numbers used may vary, they are only examples.

The number of stages refers to the number of membrane housings in the reverse osmosis system. The number specifies the number of membranes in each stage.

To illustrate what a reverse osmosis network is, here is an illustration of a three-stage array:

As this illustration shows, in a reverse osmosis array, it is the concentrate that is returned to the next stage. In this way, the percentage of water used increases, which reduces unnecessary waste.

It is therefore with the aim of optimizing the use of water that a reverse osmosis network is created. Although there are exceptions, typically, the recovery rate of the different stages is around the following percentages:

  • Stage 1: 50%.
  • Stage 2 : 25%
  • Stage 3: 12.5

This means that with a Reverse Osmosis system having a three-stage array, the water recovery rate would be around 82.5%. However, it is important to note that the installation of a third stage of treatment is not adequate for everyone since the concentration rate of the concentrate can become problematic and promote fouling of the membranes, which would affect the quality of the treatment.


How to determine the number of osmotic membranes per stage?

As mentioned above, the numbers used to define the array of a Reverse Osmosis system represent the number of membranes found in each stage of the array.

To use the example used above, in an 8:4:2 reverse osmosis network, in stage 1, there would be 8 membranes, in stage 2, 4 membranes and in stage 3, 2 membranes. To determine the membrane interval, several aspects must be taken into account.

  • The targeted flow rate;
  • The quality of the feed water;
  • The quality to be achieved;
  • The temperature range of the water; and
  • The type of membrane.

The importance of considering all of these parameters are that any small change in one of these parameters can affect the integrity of the treatment. For example, it is known that the viscosity of fluid changes with their temperature. The colder it is, the more viscous it is, which can affect the flow rate and internal pressure of a reverse osmosis system.


Case Study: Designing a 30 Gpm Reverse Osmosis System

  • Situation 1: In this situation, the water is rather cold [4°C - 8°C] and has a dissolved matter concentration of about 600 ppm. Without taking into account the type of membrane and the water quality to be achieved, the optimal solution here would be a 3-stage network that would look like this: 4 : 3 : 1


  • Situation 2: In this situation, the water is warmer [8°C - 12°C] and has a dissolved matter concentration of about 400 ppm. Without taking into account the type of membrane and the water quality to be achieved, the optimal solution here would be a 2-stage network that would look like this: 4 : 2

Just to clarify, generally speaking, the best quality permeates will be offered by reverse osmosis systems with a two-stage network.



What are the "passes" in a reverse osmosis water treatment system

When we hear about a second pass in a Reverse Osmosis system, it is a way to explain that the permeate, i.e., the treated water, will pass a second time in a different Reverse Osmosis system in order to improve the quality of the said permeate. In this way, the presence of dissolved matter and its conductivity can be lowered to exceptional levels.

Moreover, a complete water treatment system can contain a first reverse osmosis with an 8:4:2 network, for example, and a second reverse osmosis with a 6:3 network.


What are the differences between the network and the "passes" of a reverse osmosis system

Simply put, when we think of a two-pass reverse osmosis system, it is the permeate that passes through a separate reverse osmosis system once more. On the other hand, when a reverse osmosis system has several stages in its network, it is the concentrate that is directed to other osmotic membranes in the same system.

The advantage of choosing a multistage system is that it allows you to acquire a system that, in some cases, exactly meets your needs while allowing you to make significant savings. This is due in part to the fact that through the different stages, adjustments can be made to meet specific objectives.


The Reverse Osmosis Network Demystified

As you have probably understood, the importance of understanding the concepts of "stages" and "passes" in a reverse osmosis system is paramount in order to fully understand the justifications during the selection of a system. Moreover, a better understanding of your equipment will also allow you to identify more quickly if there are problems related to its operation.

In short, we hope that this information will be useful to you and that it will allow you to better understand the ramifications of reverse osmosis systems. In the meantime, here are a few articles that may be of interest to you, feel free to consult them for more details:






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